The use of undifferentiated cell cultures suffers from various constraints including:
1. Many undifferentiated cell culture systems do not display significant secondary biosynthesis and require a level of structural differentiation more closely akin to that found in the intact plant.
2. Often undifferentiated cell culture systems display considerable genetic instability during prolonged culturing. Diminished production of secondary metabolites is commonly observed.
3. Fine suspensions are often difficult to achieve, particularly ones retaining high organogenic potential. Large aggregates usually have slow growth rates; for example, A. cepa root cultures grow faster than submerged callus-like aggregates.
Freeman et al (1974) and Selby and Collin (1976) have demonstrated that callus cultures of Allium cepa accumulate only very small amounts of the desired flavor precursor compounds. The same has also been shown for Allium sativum (Malpathak and David, 1986). Redifferentiated root-like structures, however, were biosynthetically capable (Freeman, 1974). Efforts aimed at enhancing flavor precursor levels by clonal selection have failed (Selby and Collin, 1976). Likewise, previous attempts to increase the productivity of undifferentiated onion cultures by the addition of biosynthetic precursors such as cysteine, valine, sulfate, serine and methacrylic acid, have failed (Selby et al., 1980). Granroth (1970) and Turnbull et al (1981) have observed rapid uptake and transformation of labelled biosynethetic precursors by excised onion shoot tips. Krikorian and Katz (1967), demonstrated that it is possible to grow onion roots in culture in a fully differentiated state, however, no attempt was made to achieve or characterize their secondary biosynthetic potential. Klein and Edsall reported the isolation of callus cultures from seeds of Allium cepa.
Because of the limitations on the use of undifferentiated cell cultures for secondary metabolite production there is a need for an improved method for producing such secondary metabolites from organ culture and particularly root cultures. Further, there is a need to produce viable root cultures capable of yielding a uniform and genetically stable differentiated roots in high yield The present bioprocess and reactor utilizes root cultures for the growth of roots and production of flavorants and flavor precursors.